Integrated circuits, such as microprocessors and memory devices, include many metal-oxide-semiconductor field-effect transistors (MOSFETs), which provide the basic switching functions to implement logic gates, data storage, power switching, and the like. Power MOSFETs have typically been developed for applications requiring power switching and power amplification.
In one application, flyback converters, which generate a DC output from either an AC or DC input, are a simple switch mode power supply using a power MOSFET. FIG. 1 depicts a conventional flyback converter. When the MOSFET switch S turns on, energy from the input source Vin is applied and a current flows through the primary windings energizing a transformer T. The current in primary side of the transformer T ramps up proportional to the input voltage Vin. During this time, the output diode D is reverse-biased and off. The voltage applied to the diode D is equal to the output voltage Vout plus the reflected input voltage (i.e., Vin*(N2/N1)). The output capacitor C supplies the load current Io during the on-time of the MOSFET switch S.
When enough energy is stored in the primary side of transformer T, the MOSFET switch S is turned off and the energy in the transformer T transfers to the secondary side and current flows through the diode D. The diode D is now forward-biased, replenishing the energy in the output capacitor C and supplying the load. The current in the secondary side of transformer T ramps down proportionally to output voltage Vout. During this time, the primary side is considered an open circuit. The voltage applied to the MOSFET switch S is equal to the input voltage Vin plus the reflected output voltage (i.e., Vout*(N1/N2)).
Flyback converters may be operated in different modes. In one example, a fly-back converter is designed with a fixed switching frequency and modulates the peak current to supply the load demands. In another example, a fly-back converter can be operated in quasi-resonant mode (QR), where the switching occurs on the very first and deepest resonant valley. QR delivers the maximum amount of power by adjusting both the peak current and the switching frequency to turn the MOSFET switch on at the first resonant valley where VDS is at or near zero for minimal turn-on losses.
However, the benefits of flyback converters in the QR mode are reduced when used at high input voltage (e.g., over 300 V AC). This is due to the fact that higher reflected voltages require using a MOSFET with a correspondingly higher breakdown voltage rating. The use of a MOSFET with higher breakdown voltage increases the cost and inherently increases the drain-source on resistance (Rds-on) and the switching capacitance of the MOSFET. Lower breakdown MOSFETS can be used however at low load and high line voltages but the zero voltage switching capability is compromised and not achieved over the full load range. One of the proposed methods uses a cascode switch to maximize reflected voltage and utilize a MOSFET voltage level which affords the QR operation. Cascode switches typically have two or more power transistors (e.g., MOSFETs) connected in series. The load voltage is distributed across all of the series connected power transistors. As such, the use of the cascode switch increases the overall breakdown voltage without adding cost or appreciably compromising MOSFET performance.
Overvoltages have been a challenging issue for power devices (e.g., switches in fly-back converters). Overvoltages including voltage spikes from parasitic capacitances and transformer inductances, surges and fast transients, often occur during a flyback converter's normal operation and start up. Overvoltages may cause problematic field failure results of the power transistors. It is thus desirable to identify fault conditions (e.g., fault current and fault voltage) during operation of power devices and to activate suitable remedial action.
It is within this context that embodiments of the present invention arise.